Figure 1.

Blood-derived monocytes are recruited to the tumor sites, where cytokines promote their polarization towards M1 or M2 macrophages. M1 macrophages are differentiated in response to the simulation with interferon-γ (IFN−γ), Toll−like receptor ligands, such as lipopolysaccharide (LPS), or granulocyte−macrophage colony−stimulating factor. These cells produce interleukin (IL)-12, which in turn stimulates Th1 cell differentiation and activation of CD8+ T cells. Altogether these cell types limit cancer growth and diffusion. In contrast, together with regulatory T cells and myeloid-derived suppressor cells, M2 macrophages, which are differentiated in response to stimulation with IL−4, IL−10, and IL-34/MCSF-1, contribute to generate a microenvironment that sustains cancer cell behaviour.